In areas with low wave heights typically below 5 metres significant in a 100-year condition, it is often advantageous to use a so-called spread mooring instead of a turret mooring for the production vessel (FPSO) and a storage vessel (FSO).
Spread mooring means that the FPSO/FSO vessel cannot weathervane if the environmental forces caused by waves, wind and stream changes direction. This further means that the offloading of oil from the tanker often is not feasible in tandem configuration since the vessels will have different heading. Tandem configuration means that the vessels are positioned in a row with mooring rope extending between the bow on the export tanker and the aft end of the FPSO/FSO unit.
Today, export of oil from spread moored FPSO/FSO vessel often takes place via a so-called CALM buoy (Catenary Anchor Leg Mooring) which typically is positioned at a distance of two to three kilometres from the FPSO/FSO vessel. At large water depths typically from 800 metres and more, the cost for such systems will be in the order of NOK 1.000-1.500 millions. This entails large extra cost both related to the purchase of buoy, mooring, hosing and exporter from the FPSO/FSO unit to the export buoy, but also with respect to operation and maintenance.
U.S. Pat. No. 6,983,712 discloses an oil loading system and a mooring system between a floating production, storage and offloading vessel (FPSO), moored in deep water by so-called spread mooring, and a shuttle tanker moving between a terminal and the moored FSPO. A submerged yoke has one end coupled to the moored FSPO, while a buoy extending through the sea surface is moored to the opposite end of the yoke. At its middle area the yoke is moored to the sea bottom by a chain. The export tanker is rotatably moored to the buoy such that the tanker is allowed to turn dependent upon the prevailing wind, wave and stream conditions. In the loading and mooring system a mooring rope and a hose for transfer of hydrocarbons extend from the buoy to the bow of the tanker. Today export of oil from spread moored FPSO/FSO vessels typically takes place via a so-called CALM buoy which typically is placed at a distance of two to three kilometres from the FPSO/FSO vessel. At large water depth the cost for such systems will be in the order of NOK 800-1.000 million.